pollination

pollination

pollination, transfer of pollen from the male reproductive organ (stamen or staminate cone) to the female reproductive organ (pistil or pistillate cone) of the same or of another flower or cone. Pollination is not to be confused with fertilization, which it may precede by some time—a full season in many conifers. The most common agents of pollination are flying insects (as in most flowering plants) and the wind (as in many trees and all grasses and conifers), but crawling and hopping insects, snails, bats, primates, rodents, and hummingbirds may also serve. The devices that operate to ensure cross-pollination and prevent self-pollination (see sex) are varied and sometimes extremely intricate. Among them are different maturation times for the pollen and eggs of the same flower or plant, separate staminate and pistillate flowers on the same or on different plants, chemical properties that make the pollen and eggs of the same plant sterile to each other, and specialized mechanisms or structural arrangements that prevent the pollinating agent from transferring the pollen of a flower to its own stigma. In the lady's-slipper the bee enters the nectar-filled pouch by one opening and must leave by another; in so doing it brushes first past the stigma, which scrapes pollen off its back, and then past the stamens, which deposit another load of pollen. The stamens of the mountain laurel are bent back and held like springs by notches in the petals; when the bee alights it contacts the tall pistil and then, in probing deeper for nectar, triggers the stamens. Pollen is catapulted onto the insect's underside, ready for contact with the next pistil. Other examples of floral adaptations to their pollinating agents are the fig and its wasp and the yucca and its moth. Wind pollination, depending as it does on statistical chance for successful pollination, requires vast quantities of pollen, which may be forcefully ejected by the anther sac (as in grasses and ragweed) or may be exposed (as in cones and catkins) to the slightest breeze. See breeding.

Pollination is a necessary step in the sexual reproduction of flowering plants, resulting in the production of offspring that are genetically diverse. It is important in horticulture and agriculture, because fruiting is dependent on fertilisation, which is the end result of pollination.

Types

The process of pollination requires pollinators: agents that carry or move the pollen grains from the anther to the receptive part of the carpel or pistil. The receptive part of the carpel is called a stigma in the flowers of angiosperms. The receptive part of the gymnosperm ovule is called the micropyle. The various flower traits that attract different pollinators are known as pollination syndromes. Methods of pollination, with common pollinators or plants, are:

Abiotic pollination occurs when pollination is mediated without the involvement of other organisms. Only 10% of flowering plants are able to pollinate without animal assistance. For example, anemophily is pollination by wind. This form of pollination is very common in grasses, most conifers, and many deciduous trees. Hydrophily is pollination by water and occurs in aquatic plants which release their seeds directly into the surrounding water. About 80% of all plant pollination is biotic. Of the 20% of abiotically pollinated species, 98% is by wind and 2% by water.

In agriculture

Pollination management is a branch of agriculture that seeks to protect and enhance present pollinators and often involves the culture and addition of pollinators in monoculture situations, such as commercial fruit orchards. The largest managed pollination event in the world is in Californianalmond orchards, where nearly half (about one million hives) of the US honey bees are trucked to the almond orchards each spring. New York's apple crop requires about 30,000 hives; Maine's blueberry crop uses about 50,000 hives each year.

The ecological and financial importance of natural pollination by insects to agriculturalcrops, improving their quality and quantity, becomes more and more appreciated and has given rise to new financial opportunities. The vicinity of a forest or wild grasslands with native pollinators near agricultural crops, such as apples, almonds or coffee can improve their yield by about 20%. The benefits of native pollinators may result in forest owners demanding payment for their contribution in the improved crop results - a simple example of the economic value of ecological services.

The US solution to the pollinator shortage, so far, has been for commercial beekeepers to become pollination contractors and to migrate. Just as the combine harvesters follow the wheatharvest from Texas to Manitoba, beekeepers follow the bloom from south to north, to provide pollination for many different crops.

Pollinators and pollenizers

Pollination also requires consideration of pollenizers. (The terms "pollinator" and "pollenizer" are often confused: a pollinator is the agent that moves the pollen, whether it be wind, bees, bats, moths, or birds; a pollenizer is the plant that provides the pollen.) Some plants are self-fertile or self-compatible and can pollinate themselves. Other plants have chemical or physical barriers to self-pollination and need to be cross-pollinated: with these self-infertile plants, not only pollinators must be considered but pollenizers as well. In pollination management, a good pollenizer is a plant that provides compatible, viable and plentiful pollen and blooms at the same time as the plant that is to be pollinated.

Pollination can be cross-pollination with a pollinator and an external pollenizer, self-pollenization with a pollinator, or self-pollination without any pollinator:

Cross-pollination (syngamy): pollen is delivered to a flower of a different plant. Plants adapted to outcross or cross-pollinise have taller stamens than carpels to better spread pollen to other flowers.

Self-pollenization (autogamy): pollen moves to the female part of the same flower, or to another flower on the same individual plant. This is sometimes referred to as self-pollination, but this is not synonymous with autogamy. Clarity requires that the term "self-pollination" be restricted to those plants that accomplish pollination without an external pollinator (example: the stamens actually grow into contact with the pistil to transfer the pollen). Most peach varieties are autogamous, but not truly self-pollinated, as it is generally an insect pollinator that moves the pollen from anther to stigma. Plants adapted to self-fertilize have similar stamen and carpel length.

Cleistogamy: pollination that occurs before the flower opens is always self-pollination. Some cleistogamous flowers never open, in contrast to chasmogamous flowers that open and are then pollinated. Cleistogamous flowers must of necessity be self-compatible or self-fertile plants. Other plants are self-incompatible. These are end points on a continuum, not absolute points.

Hybridization is effective pollination between flowers of different species of the same genus, or even between flowers of different genera (as in the case of several orchids).

Peaches are considered self-fertile because a commercial crop can be produced without cross-pollination, though cross-pollination usually gives a better crop. Apples are considered self-incompatible, because a commercial crop must be cross-pollinated. Remember that most fruits are graftedclones, genetically identical. An orchard block of apples of one variety is in effect all one plant. Growers now consider this a mistake. One means of correcting this mistake is to graft a limb of an appropriate pollenizer (generally a variety of crabapple) every six trees or so.

Honey bee pollination

Honey bees travel from flower to flower, collecting nectar (later converted to honey), and pollen grains. The bee collects the pollen by rubbing against the anthers. The pollen collects on the hind legs, in dense hairs referred to as a pollen basket. As the bee flies from flower to flower, some of the pollen grains are transferred onto the stigma of other flowers.

Nectar provides the energy for bee nutrition; pollen provides the protein. When bees are rearing large quantities of brood (beekeepers say hives are "building"), bees deliberately gather pollen to meet the nutritional needs of the brood. A honey bee that is deliberately gathering pollen is up to ten times more efficient as a pollinator than one that is primarily gathering nectar and only unintentionally transferring pollen.

Good pollination management seeks to have bees in a "building" state during the bloom period of the crop, thus requiring them to gather pollen, and making them more efficient pollinators. Thus the management techniques of a beekeeper providing pollination services are different from, and somewhat incompatible with, those of a beekeeper who is trying to produce honey.

Other species of bees differ in various details of their behavior and pollen-gathering habits, and it should be remembered that honey bees are not native to the Western Hemisphere; all pollination of native plants in the Americas has been historically performed by various native bees.